server.py

#!/usr/bin/env python

from twisted.internet.protocol import Protocol, Factory
from twisted.internet import reactor

class Scream(Protocol):
    version = b'123'
    def dataReceived(self, data):
        self.transport.write(b'AAAAAAAA' + self.version)

if __name__ == '__main__':
    f = Factory()
    f.protocol = Scream
    reactor.listenTCP(12346, f)
    reactor.run()

Run with

$ python3 -m venv .venv
$ source .venv/bin/activate
$ pip install twisted
$ python server.py